1. Field of the Invention
The present invention relates to a sputtering target, an optical thin film and a manufacturing method thereof using the sputtering target, and an optical recording medium.
2. Description of the Related Art
A SiC thin film is used in a dielectric layer, a protective layer, a light absorbing layer, and so on of a conventional optical recording medium. However, the SiC thin film is opaque for light in the visible spectrum and exhibits a low transmittance for light even in the red spectrum. Further, the SiC thin film absorbs much of light in the blue-violet spectrum used for a next-generation high-density optical disk and thus scarcely transmits the light.
In a next generation high-density rewritable optical disk using a blue-violet laser, effective is a so-called low-to-high polarity medium which exhibits a higher reflectance when data is recorded thereon than when no data is recorded thereon. A known approach to achieve a low-to-high polarity optical disk is the following film structure. That is, in a structure having a first dielectric film, a phase-change recording film, a second dielectric film, and a reflection film stacked in the order from a light incident side, the first dielectric film is constituted of a multilayer dielectric film having a high refractive-index dielectric film, a low refractive-index dielectric film, and a high refractive-index dielectric film
A SiO2 film is generally used as the low refractive-index dielectric film since the lower the refractive index of the low refractive-index dielectric film is, the better. Known film deposition methods of the SiO2 film are a sputtering method using a SiO2 target, a reactive sputtering method using a Si target, and the like. The former method, however, has problems of a very low film deposition rate and a great increase in substrate temperature. The latter method has a problem that conductivity is lowered due to the oxidization of a surface of the Si target during the sputtering, which inhibits the continuation of stable sputtering. Because of these reasons, the sputtering method of using the SiO2 target and the reactive sputtering method using the Si target are not suitable for mass production.
Meanwhile, a SiOC film formed by reactively sputtering a sputtering target containing Si and C as its major components in anoxygen-containing rare gas atmosphere is known as being transparent and having a low refractive index, though still having a slight absorption coefficient for light in the blue-violet spectrum (see Japanese Patent Laid-open Application No. 2005-025851). Owing to its high deposition rate and excellent mass productivity, the SiOC film formed by using the target containing Si and C as its major components is expected as a low refractive-index dielectric film of an optical recording medium. However, since sufficient consideration has not been given to its texture, composition, and the like, a conventional SiC target has a problem of easily cracking and easily causing abnormal discharge, which leads to a defect of the film. The abnormal discharge will be a cause of variation in characteristics among media.
One of possible reasons for this may be that SiC is a very high melting-point material and is thus poor in sinterbility. Accordingly, the SiC target is difficult to have a high density structure, and therefore has alow strength to easily crack. Further, the produced sputtering target has a coarse texture, which tends to cause abnormal discharge starting from pores in the target. Known approaches to achieve the higher density of the Si target are a method of sintering SiC raw material powder under conditions of a high temperature of about 2000° C. and a high pressure, and a method of adding a sintering aid to the SiC raw material powder. The former method, however, requires high cost, which will be a cause of increasing manufacturing cost of the SiC target.
The latter method of achieving the higher density uses metal elements such as Al and Fe as the sintering aid. Meanwhile, raw material powder of SiC commonly available has purity of about 2N to 3N, and the raw material powder contains, besides Si and C being the major components, elements functioning as the sintering aid and elements not having such a function. Therefore, if no consideration is given to the kind and amount of impurity elements in the target, the impurity elements are very much likely to be a starting point of the abnormal discharge. As a result, the abnormal discharge of ten occurs during the sputtering.
Moreover, if the target contains too large an amount of the sintering aid, components (metal elements such as Al and Fe) of the sintering aid are excessively taken in the film deposited by the sputtering, which increases the absorption of the SiOC film. The use of such a SiOC film in an optical recording medium causes a problem of an insufficient signal strength. In order to solve the problems ascribable to the conventional SiC target, the texture, Purity, manufacturing method, and the like of the target have to be improved, but at present, no industrially effective solution has been found yet.
A next generation phase-change optical recording medium adopts a method of narrowing a track pitch in order to improve its recording density. Such a phase-change optical recording medium has a problem that so-called cross erase is likely to occur. The cross erase refers to a phenomenon that in an area where the track pitch is substantially equal to or smaller than the size of an optical beam, data recorded to a certain track is deteriorated when data is recorded to or erased from an adjacent track. A possible cause of the occurrence of the cross erase is that the relevant track is also exposed to an edge of an optical beam with a considerably high light intensity when the adjacent track is irradiated with the optical beam, and only an effect of the light irradiation deteriorates recorded marks on the relevant track. Another possible cause may be that heat generated when the adjacent track is heated by the optical beam is transferred to the relevant track by heat transfer in an inward direction on a film plane, and the influence thereof deteriorates the shape of the recorded marks.
An effective solution to the cross erase due to the former cause is a film structure in which a light absorption ratio Aa of an amorphous portion (recorded portion) is made smaller than a light absorption ratio Ac of a crystalline portion (unrecorded portion). In order to satisfy Aa<Ac, the adoption of the film structure having the aforesaid low-to-high polarity is effective. The aforesaid patent publication describes this film structure and further describes that adopting a SiOC film whose C concentration is 0.1 to 30 atom % as the low refractive-index dielectric film achieves the low-to-high signal polarity and improves thermal conductivity of the film owing to carbon(C). The improved thermal conductivity of the SiOC film enables the heat caused by the laser beam irradiation to be effectively transferred in a direction perpendicular to the film plane. As a result, the cross erase can be further reduced.
The aforesaid patent publication describes that the SiOC film whose C concentration is within the range from 0.1 atom % to 30 atm % exhibits optical characteristics of 1.45<n <1.55 and k<0.01 for light in the blue-violet wavelength spectrum when a complex refractive index is expressed as n-ik (n: refractive index, k: attenuation coefficient). However, the attenuation coefficient k of 0.01 or lower is not small enough to obtain an optical recording medium having a sufficient reflectance contrast. Therefore, there is a demand for a dielectric film which maintains the effect of reducing the cross erase on one hand and has a further reduced attenuation coefficient k on the other.
The SiOC film is applicable not only to the low refractive-index dielectric film of the phase-change optical recording medium but also to optical thin films such as a dielectric film of a write-once optical recording medium, and an antireflection film, a selective transmissive film, an interference film, a polarizing film, and the like of various kinds of parts and members. For example, in a write-once optical recording medium that records data by an irreversible optical change, an organic dye is mainly used as a material of a recording film. Generally, an organic dye film (recording film) is formed on a resin substrate such as a polycarbonate substrate, and a metal reflection film is further provided thereon. However, if the metal reflection film in contact with the organic dye is thin, sufficient thermal conductivity cannot be obtained. This-poses a problem that heat caused by an applied laser pulse does not quickly diffuse, so that the shape of the recorded marks is deteriorated.
Another problem is that, when the organic dye is applied on the substrate, the polycarbonate substrate is influenced by a solvent which is blended for improving coatability of the organic dye, so that the recording film is gradually deteriorated when it is kept under a high-temperature environment. Further, when a so-called blue-violet laser with a wavelength of about 405 nm is used as a light source, a silver-series material is best suited for sufficiently increasing a reflectance. However, manufacturing cost of an optical disk is increased because silver is expensive. Reducing the thickness of the reflection film with in order to cut down the manufacturing cost causes problems of deteriorated thermal conductivity and deteriorated shape of the recorded marks. In addition, the recording film made of the organic dye is directly formed on a surface of the substrate by a spin coating method of the like. Therefore, it has a problem that, when it is kept under high-temperature and higher-humidity conditions for a long time, the solvent in the organic dye reacts with the substrate to cause gradual deterioration of its recording characteristic.
An effective solution to these problems is to provide a low refractive-index SiOC film between the resin substrate and the organic dye film (recording layer). The SiOC film has a refractive index approximate to that of the resin substrate and thus unnecessary light reflection on an interface is small, so that optical problems do not easily occur. From a viewpoint of mass productivity, the SiOC film is also preferably formed by reactively sputtering a sputtering target containing Si and C as its major components in an oxygen-containing gas atmosphere, similarly to the SiC film used in the phase-change optical recording medium. However, this still has a problem that characteristic variation among media is likely to occur due to the abnormal discharge ascribable to the conventional SiC target.
The adoption of the SiOC film as an optical thin film such as an antireflection film, a light-selective transmissive film, an optical interference film, or a polarizing film has a problem that the impurity elements contained in the conventional SiC target are excessively taken in the film deposited by the sputtering, so that the absorption is increased. Since the impurities in the film originating in the conventional SiC target include the sintering aid necessary for achieving the higher density of the target, the absorption of the film greatly increases, so that the film transmits substantially no light in the ultraviolet spectrum.